In the technical field of internal combustion engines, it is known to place a water temperature sensor downstream of an outlet of a water jacket formed in an engine main body. See US2006/0042566A1, for instance. In this prior art, a passage forming member internally defining both a passage leading to a radiator and a passage bypassing the radiator is attached to a cooling water outlet of an engine main body, and a water temperature sensor is positioned in a part of the passage forming member where the two passages branch off from each other.
However, when a water temperature sensor is provided in a branching part where a passage leading to the radiator and a passage bypassing the radiator branch off from each other, because the state of the water flow at the branching part may vary depending on if the cooling water is flowing into the radiator or if the water flow is bypassing the radiator, the accuracy of temperature detection may not be ensured depending on the situation. When the cooling water does not flow from the water jacket into the radiator, the cooling water held in the radiator and having a relatively low temperature may flow backward to the branching part so that the temperature detected by the water temperature sensor may be lower than the actual temperature of the cooling water flowing out of the water jacket.